Cell-surface properties of enterotoxigenic and cytotoxic Salmonella enteritidis and Salmonella typhimurium: studies on hemagglutination, cell-surface hydrophobicity, attachment to human intestinal cells and fibronectin-binding

Interactions of Salmonella with animals and plants

Salmonella enterica species are Gram-negative bacteria, which are responsible for a wide range of food- and water-borne diseases in both humans and animals, thereby posing a major threat to public health. Recently, there has been an increasing number of reports, linking Salmonella contaminated raw vegetables and fruits with food poisoning. Many studies have shown that an essential feature of the pathogenicity of Salmonella is its capacity to cross a number of barriers requiring invasion of a large variety of cells and that the extent of internalization may be influenced by numerous factors. However, it is poorly understood how Salmonella successfully infects hosts as diversified as animals or plants. The aim of this review is to describe the different stages required for Salmonella interaction with its hosts: (i) attachment to host surfaces; (ii) entry processes; (iii) multiplication; (iv) suppression of host defense mechanisms; and to point out similarities and differences between animal and plant infections.

Reducing colonization of Salmonella Enteritidis in chicken by targeting outer membrane proteins

AIMS

To evaluate the ability of Salmonella enterica ser. Enteritidis outer membrane proteins (OMPs) of 75.6 and 82.3 kDa to inhibit or reduce in vivo colonization of S. Enteritidis on intestinal mucosa in chickens.

METHODS AND RESULTS

Nine-week-old specific-pathogen-free chickens were subcutaneously immunized with 75.6 or 82.3 kDa protein, and challenged with a virulent strain of S. Enteritidis. Chickens were killed, and portions of small intestine and caecum were removed at necropsy. The population of S. Enteritidis attached to chicken intestinal mucosa was determined. The population of S. Enteritidis recovered from the small intestine and caecum of chickens immunized with 75.6 or 82.3 kDa protein was significantly (P < 0.05) lower than that recovered from the control birds.

CONCLUSIONS

Salmonella Enteritidis OMPs 75.6 kDa and 82.3 kDa were effective in reducing colonization of S. Enteritidis on intestinal mucosa in chickens.

SIGNIFICANCE AND IMPACT OF THE STUDY

Salmonella Enteritidis OMPs 75.6 or 82.3 kDa could be used as potential vaccines to reduce S. Enteritidis colonization in chickens.

Identification of Salmonella enteritidis outer membrane proteins expressed during attachment to human intestinal epithelial cells

AIMS

To express and identify the Salmonella enterica ser. Enteritidis outer membrane proteins that are involved in the attachment to human intestinal epithelial cells.

METHODS AND RESULTS

Salmonella Enteritidis attachment proteins were expressed by infection of human intestinal epithelial cells (int-407) with Salm. Enteritidis strain SE28, followed by pulse labelling with [35S]-methionine. Outer membrane proteins were separated by TritonX-114 phase partitioning, and were detected by Salm. Enteritidis-specific polyclonal antibody. Outer membrane proteins of molecular weights 82.3, 75.6, 49.3, 35.5 and 19.3 kDa were newly synthesized. Expression of 31.2 and 16 kDa proteins was up-regulated, whereas the expression of 40.7 kDa protein was down-regulated. Polyclonal antibodies against the 82.3 and 75.6 kDa proteins significantly (P < 0.05) reduced the binding of Salm. Enteritidis to int-407 cells in vitro.

CONCLUSIONS

Outer membrane proteins 82.3 and 75.6 kDa are potentially involved in the attachment of Salm. Enteritidis to the intestinal mucosa.

SIGNIFICANCE AND IMPACT OF THE STUDY

Outer membrane proteins 82.3 and 75.6 kDa identified in this study could be used as potential vaccines to block or reduce Salm. Enteritidis colonization in chickens.

In vitro attachment and invasion of chicken ovarian granulosa cells by Salmonella enteritidis phage type 8

The attachment and invasion of chicken ovarian granulosa cells by Salmonella enteritidis was examined in vitro. The attachment was inhibited by preincubation of granulosa cells with anti-chicken fibronectin antibody (approximately 70% reduction in attachment) or preincubation with a 14-kDa fimbrial protein isolated from S. enteritidis (68% reduction in attachment). Treatment of bacterial cells with the tetrapeptide RGDS before addition to granulosa cells resulted in inhibition of attachment (60% inhibition when 2 x 10(7) CFU of bacteria was treated with 500 microg of peptide). Treatment with the peptide GRGD resulted in similar magnitude of inhibition, indicating that extracellular matrix proteins play significant roles in the interaction of S. enteritidis with granulosa cells. In contrast, treatment of the bacterial cells with the peptide GRAD did not result in significant inhibition of attachment to the granulosa cells. S. enteritidis was found to attach specifically to fibronectin, collagen IV, and laminin-coated microtiter plate wells, with the rank order of attachment as follows: fibronectin > laminin > collagen IV. Light and transmission electron micrographs of S. enteritidis invasion of granulosa cells showed organisms with or without a surrounding membrane in the cytoplasm of granulosa cells. In some instances, dividing bacterial cells were observed in the cytoplasm. Results of this study demonstrated that S. enteritidis interacts with granulosa cells in a specific manner and can invade and multiply in these cells. The granulosa cell layer of the preovulatory follicles may be a preferred site for the colonization of the chicken ovaries by invasive strains of S. enteritidis.

Experimental infection of laying hens with Salmonella enteritidis strains that express different types of fimbriae

A study was conducted to compare the pathogenicity of three Salmonella enteritidis phage type 8 strains (9, 21, and 30) in 30-wk-old laying hens. Strain 9 expressed two types of fimbriae of 14 and 21 kDa. Strain 30 expressed a single fimbrial type (21 kDa). Strain 21 did not express any fimbrial protein. Laying hens were divided into three groups of 35 each and each group was orally inoculated with a single S. enteritidis strain (1 x 10(8) cfu per bird). Significantly less intensive cecal colonization and fecal shedding of the organism were observed in hens that were inoculated with the strain that did not express fimbriae than in birds inoculated with other two strains (P < 0.05). Isolation of S. enteritidis from liver, spleen, reproductive organs, and egg contents did not differ between groups. Mean serum S. enteritidis lipopolysaccharide-specific antibody titers of birds inoculated with strain 21 were lower than titers of hens that were inoculated with the other two strains from the 5th wk through the end of the trial. Immunoblot of the bacterial outer membrane structures revealed the presence of serum antibodies against lipopolysaccharide, membrane-associated proteins, and purified 14 kDa fimbrial protein in birds inoculated with strain 9 as late as 9 wk postinoculation. Results of this study are consistent with a role for fimbrial proteins in the cecal colonization by S. enteritidis. In addition, cecal colonization mediated by fimbrial proteins may enhance the elicitation of humoral immune response against S. enteritidis.

Preservative-mediated changes of the surface properties of Escherichia coli

The effectiveness of two commonly used preservatives, sodium benzoate and potassium disulfite, was evaluated in terms of their bactericidal activity and capacity to induce changes in the surface properties of Escherichia coli isolated from commercial food preserves. Preservative treatment over a five-week test period resulted in controlling the multiplication of these organisms and causing a decline in cell-surface hydrophobicity, hemagglutinating ability and adherence capacity to rat intestinal cells of E. coli isolates. A loss in motility was also exhibited.

Association with MDCK epithelial cells by Salmonella typhimurium is reduced during utilization of carbohydrates

Association of Salmonella typhimurium with MDCK epithelial cells in monolayers, represented primarily by intracellular bacteria after 30 min of contact, with centrifugation followed by vigorous washing, was measured during aerobic and anaerobic growth of the bacteria in brain heart infusion broth. Cell association was greatest during a short period in the late log phase of growth under aerobic conditions. At this time, the pH of the growth medium was changing from acid to alkaline and glucose (0.2% initially) was exhausted. Addition of excess glucose (0.5%) to brain heart infusion broth, which was not exhausted before the bacteria entered the stationary phase of growth, in which cell association dropped sharply, resulted in repression of cell association by the bacteria. The repressive effect of glucose on cell association could not be reversed by exogenous cyclic AMP in the bacterial growth medium. Under anaerobic conditions, the effect of glucose on cell association by the bacteria was not as great and the glucose was not exhausted before the bacteria entered the stationary phase. When S. typhimurium was grown in a rich but carbohydrate-free medium, cell association by the bacteria increased earlier in the growth cycle under both aerobic and anaerobic conditions. The addition of glucose and certain other utilizable carbohydrates to this medium caused a repression of cell association by S. typhimurium that was greater under aerobic growth conditions. These results show that cell association by S. typhimurium, which is accompanied by rapid internalization (cell invasion), is the same under aerobic and anaerobic conditions if the bacteria are grown to the log phase in a carbohydrate-free medium. This suggests that prior reports of greater cell invasion by S. typhimurium during anaerobic growth may have arisen from the use of media containing carbohydrates which were found to be more repressive during aerobic growth of the bacteria.

Fibronectin binding by Salmonella strains: evaluation of a particle agglutination assay

Thirty-five Salmonella strains isolated from human cases of salmonellosis were tested and compared for their fibronectin (fn) binding capacities by using two fn-particle agglutination assays (fn-PAAs) prepared by coating with human fn either (i) latex beads (Difco; 0.81-micron diameter) (L-fn-PAA) or (ii) heat-killed formalin-treated Staphylococcus aureus Cowan 1 cells (C-fn-PAA). Six S. aureus strains were also included in this study as controls. The strains were cultured on colonization factor antigen agar and blood agar and in tryptic soy broth and brain heart infusion broth. The Salmonella and S. aureus strains were cultured at 33 and 37 degrees C, respectively, for optimal expression of fn-binding proteins. Bacterial cells (approximately 10(10) cells per ml) harvested from growth in various culture media and suspended in 0.02 M potassium phosphate buffer (pH 6.8) agglutinated the fn-PAA reagents. These reactions were scored semiquantitatively from + to + depending on the speed or intensity of the reactions within 2 min. Maximum agglutination in fn-PAA systems was observed when the cells were grown in brain heart infusion broth, while tryptic soy broth proved to be least suitable media for culturing cells for fn-PAAS. Although a statistically highly significant correlation was obtained between results of assays of radiolabeled fn and 29-kDa fragment binding, no significant correlation was observed (i) between the results of strains cultured in different media or (ii) when semiquantitative score results of the two fn-PAA systems were compared with those of the conventional radiolabeled fn assay. To enhance the efficiency of the test system, the C-fn-PAA reagent was stained with methylene blue (2% in 0.17 M glycine-NaOH buffer [pH 6.8]). This facilitated easy interpretation of results, which could be performed on hydrophobic paper instead of glass slides. The results obtained with both unstained C-fn-PAA and stained C-fn-PAA were comparable to each other and reproducible. Although the fn-PAAs are simple and easy to perform, the results did not differentiate between negative, low, moderate, and high binding abilities when Salmonella strains were evaluated for fn binding, and the results were not comparable to those obtained by the conventional radiolabeling method.

Toxin profiles, and cell-surface properties of Salmonella strains isolated from Swedish travelers

Toxin production, cell-surface hydrophobicity and fibronectin-binding properties of 21 Salmonella strains of different species, isolated from Swedish travelers to different parts of the world, were studied. Cell sonicate supernatants from blood agar grown cultures of 80% of the strains induced rabbit skin permeability reaction in the form of induration and/or blueing while 33% of the strains also produced cell necrotizing factor on rabbit skin. Four strains were negative in the rabbit skin permeability test, while only two were negative when tested on CHO cells. When cultured on blood agar, a majority of the strains (17/21) showed low cell-surface hydrophobicity, showing no aggregation even at 1.5 M ammonium sulfate concentration in salt aggregation test (SAT), while only four strains showed high cell-surface hydrophobicity. Furthermore, these strains could be classified as low fibronectin binders due to their poor interaction with fibronectin or its 29 kDa N-terminal fragment.

Purification and characterization of thin, aggregative fimbriae from Salmonella enteritidis

Novel fimbriae were isolated and purified from the human enteropathogen Salmonella enteritidis 27655. These fimbriae were thin (measuring 3 to 4 nm in diameter), were extremely aggregative, and remained cell associated despite attempts to separate them from blended cells by centrifugation. The thin fimbriae were not solubilized in 5 M NaOH or in boiling 0.5% deoxycholate, 8 M urea, or 1 to 2% sodium dodecyl sulfate (SDS) with or without 5% beta-mercaptoethanol. Therefore, an unconventional purification procedure based on the removal of contaminating cell macromolecules in sonicated cell extracts by enzymatic digestion and preparative SDS-polyacrylamide gel electrophoresis (PAGE) was used. The insoluble fimbriae recovered from the well of the gel required depolymerization in formic acid prior to analysis by SDS-PAGE. Acid depolymerization revealed that the fimbriae were composed of fimbrin subunits, each with an apparent molecular mass of 17 kDa. Although their biochemical characteristics and amino acid composition were typical of fimbriae in general, these thin fimbriae were clearly distinct from other previously characterized fimbriae. Moreover, their fimbrin subunits had a unique N-terminal amino acid sequence. Native fimbriae on whole cells were specifically labeled with immune serum raised to the purified fimbriae. This immune serum also reacted with the denatured 17-kDa fimbrin protein in Western blots. The polyclonal immune serum did not cross-react with the other two native fimbrial types produced by this strain or with their respective fimbrins on Western blots (immunoblots). Therefore, these fimbriae represent the third fimbrial type produced by the enteropathogen S. enteritidis.

Mannose-resistant adhesion of motile Aeromonas to INT407 cells and the differences among isolates from humans, food and water

Aeromonas has been recognized as an important enteropathogen, but factors related to its virulence have not been clarified. For most enteric pathogens, attachment is a prerequisite for infection and for the effective delivery of toxins to the intestinal epithelial cells. We examined a total of 273 strains isolated from stool, food and environmental specimens by an assay for mannose-resistant adhesion to INT407 cells in vitro. Seventeen of the 102 faecal isolates were adhesive strains with more than 10 bacteria adhering per cell, while only 2 of the 118 isolates from foods and river water adhered to the cells (P less than 0.001). It is possible that the adhesion might serve as a marker for discrimination between the pathogenic and nonpathogenic isolates. The 8 highly adhesive strains with more than 20 adhering organisms per cell were scrutinized for the mechanism of adhesion. No correlation was apparent between the adhesion to INT407 cells and hydrophobicity. It was noted that fucose inhibited the adhesion of four strains as well as haemagglutination by them. Electronmicroscopic studies showed the presence of flexible and curvilinear fimbriae in only 2 of the 8 highly adhesive strains.

Adherence and pathogenesis of Salmonella enteritidis in mice

Adherence of many pathogenic organisms to the host cells has been associated with the presence of fimbriae. The exact role of these organelles in the adherence and pathogenesis of Salmonella enteritidis is not well established. Utilizing hemagglutination tests, S. enteritidis was shown to possess type 1 and type 3 fimbriae. Polyacrylamide gel electrophoresis of the isolated fimbriae showed that type 1 and 3 fimbriae of S. enteritidis had subunit M.r of 17 and 22 kDa, respectively. In vitro adherence assays suggested that S. enteritidis utilized type 1 fimbriae to adhere to human buccal and mouse small intestine epithelial cells. In addition, antibody produced against type 1 and type 3 fimbriae protected the mice from infection with a lethal dose of S. enteritidis. These results suggest that type 1 and possibly type 3 fimbriae are involved in the adherence and pathogenesis of S. enteritidis. The data further suggest that they may have a role in the adherence and pathogenesis of the other enteric organisms.

The ability of Salmonella to enter mammalian cells is affected by bacterial growth state

We have examined the effect of different growth conditions on the ability of Salmonella to interact with Madin-Darby canine kidney cells. Two growth conditions that affect the expression of Salmonella adherence and invasiveness have been identified. First, bacteria lose their invasiveness in the stationary phase of growth. Second, bacteria growing in oxygen-limited growth conditions are induced for adherence and invasiveness, whereas those growing aerobically are relatively nonadherent and noninvasive. Salmonella from cultures aerated with gas mixtures containing 0% or 1% oxygen were 6- to 70-fold more adherent and invasive than those from cultures aerated with a gas mixture containing 20% oxygen. The Salmonella typhimurium oxrA gene that is required for the anaerobic induction of many proteins is not involved in the regulation of Salmonella invasiveness. We speculate that oxygen limitation might be an environmental cue that triggers the expression of Salmonella invasiveness within the intestinal lumen and other tissues.